Epsilon Ophiuchi

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Epsilon Ophiuchi
Diagram showing star positions and boundaries of the Ophiuchus constellation and its surroundings
Cercle rouge 100%.svg

Location of ε Ophiuchi (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ophiuchus
Right ascension 16h 18m 19.28974s[1]
Declination –04° 41′ 33.0345″[1]
Apparent magnitude (V) 3.220[2]
Characteristics
Spectral type G9.5 IIIb[3]
U−B color index +0.762[2]
B−V color index +0.972[2]
Astrometry
Radial velocity (Rv) –10.3[4] km/s
Proper motion (μ) RA: +83.40[1] mas/yr
Dec.: +40.58[1] mas/yr
Parallax (π) 30.64 ± 0.20[1] mas
Distance 106.4 ± 0.7 ly
(32.6 ± 0.2 pc)
Absolute magnitude (MV) +0.55[5]
Details
Mass 1.85 ± 0.05[6] M☉
Radius 10.39 ± 0.07[6] R☉
Luminosity 54[7] L☉
Surface gravity (log g) 2.59 ± 0.08[3] cgs
Temperature 4,918 ± 28[3] K
Metallicity [Fe/H] –0.13 ± 0.06[3] dex
Rotational velocity (v sin i) 5.7[8] km/s
Age 0.95–1.01[9] Gyr
Other designations
Eps Oph, 2 Ophiuchi, BD–04 4086, FK5 605, HD 146791, HIP 79882, HR 6075, SAO 141086.[10]

Epsilon Ophiuchi (ε Oph, ε Ophiuchi) is a red giant[6] star in the constellation Ophiuchus. It has the traditional name Yed Posterior. The star Delta Ophiuchi, with which it forms a naked eye optical double, is named Yed Prior. The name Yed comes from Arabic for "the hand".

Properties

Epsilon Ophiuchi is located less than five degrees south of the celestial equator in the eastern part of the constellation.[1] With an apparent visual magnitude of 3.220,[2] this allows the star to be seen with the naked eye from most of the Earth under suitably dark skies. Parallax measurements yield an estimated distance of 106.4 light-years (32.6 parsecs). It has a stellar classification of G9.5 IIIb, with the luminosity class of III indicating that this is a giant star that has exhausted the hydrogen and evolved away from the main sequence. This red giant has nearly double the Sun's mass and has expanded to an estimated radius of over ten times the radius of the Sun,[9] giving it a luminosity of about 54 times the Sun.[7] It is about a billion years old.[9]

Unusually for a class G giant, it is cyanogen-deficient and carbon-deficient.[citation needed] The outer envelope of this star displays solar-type oscillations with a period of 0.19 days, allowing the methods of asteroseismology to be applied.[6] However, the models for this star have not been able to distinguish whether this star is generating energy by the thermonuclear fusion of hydrogen along a shell, or the fusion of helium at its core. Either mode produces a good fit to the star's physical properties.[6] The projected rotational velocity of the star is 5.7 km s−1, and the inclination of the rotation axis to the line of sight from the Earth lies in the range of 41–73°.[8]

Etymology

The traditional name Yed is derived from Arabic meaning "the hand", and the two stars are the left hand of Ophiuchus (the Serpent Bearer) that holds the head of the serpent Serpens Caput. It was a member of indigenous Arabic asterism al-Nasaq al-Yamānī, "the Southern Line" of al-Nasaqān "the Two Lines",[11] along with α Ser (Unukalhai), δ Ser (Qin, Tsin), ε Ser (Ba, Pa), δ Oph (Yed Prior), ζ Oph (Han) and γ Oph (Tsung Ching).[12]

According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, al-Nasaq al-Yamānī or Nasak Yamani were the title for two stars :δ Ser as Nasak Yamani I and ε Ser as Nasak Yamani II (exclude this star, α Ser, δ Oph, ζ Oph and γ Oph).[13]

In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which is represent eleven old states in China which is marking the right borderline of the enclosure, consisting of ε Ophiuchi, β Herculis, γ Herculis, κ Herculis, γ Serpentis, β Serpentis, α Serpentis, δ Serpentis, ε Serpentis, δ Ophiuchi and ζ Ophiuchi.[14] Consequently, ε Ophiuchi itself is known as 天市右垣十 (Tiān Shì Yòu Yuán shí, English: the Tenth Star of Right Wall of Heavenly Market Enclosure), represent the state Chu (楚) (or Tsoo),[15][16][17] together with φ Capricorni (or 24 Capricorni in R.H.Allen's version[18]) in Twelve States (asterism).

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ 2.0 2.1 2.2 2.3 Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 3.2 3.3 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ 7.0 7.1 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 8.0 8.1 Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ 9.0 9.1 9.2 Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  15. ↑ Star Names - R.H.Allen p.302
  16. ↑ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
  17. ↑ (Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name, Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. ↑ Star Names - R.H.Allen p.142

External links

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